Re: [PATCH] Runtime memory barrier patching

[Ulrich Drepper <drepper@redhat.com>]

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Linus Torvalds wrote:

> They may _work_ for intel, but quite frankly they suck for most Intel (and 
> probably non-intel too) CPU's. Using prefixes tends to almost always mess 
> up the instruction decoders on most CPU's out there.

Indeed, using prefixes is terrible.  This is what is used in gas:


    {0x90};                                     /* nop                  */
  static const char f32_2[] =
    {0x89,0xf6};                                /* movl %esi,%esi       */
  static const char f32_3[] =
    {0x8d,0x76,0x00};                           /* leal 0(%esi),%esi    */
  static const char f32_4[] =
    {0x8d,0x74,0x26,0x00};                      /* leal 0(%esi,1),%esi  */
  static const char f32_5[] =
    {0x90,                                      /* nop                  */
     0x8d,0x74,0x26,0x00};                      /* leal 0(%esi,1),%esi  */
  static const char f32_6[] =
    {0x8d,0xb6,0x00,0x00,0x00,0x00};            /* leal 0L(%esi),%esi   */
  static const char f32_7[] =
    {0x8d,0xb4,0x26,0x00,0x00,0x00,0x00};       /* leal 0L(%esi,1),%esi */
  static const char f32_8[] =
    {0x90,                                      /* nop                  */
     0x8d,0xb4,0x26,0x00,0x00,0x00,0x00};       /* leal 0L(%esi,1),%esi */
  static const char f32_9[] =
    {0x89,0xf6,                                 /* movl %esi,%esi       */
     0x8d,0xbc,0x27,0x00,0x00,0x00,0x00};       /* leal 0L(%edi,1),%edi */
  static const char f32_10[] =
    {0x8d,0x76,0x00,                            /* leal 0(%esi),%esi    */
     0x8d,0xbc,0x27,0x00,0x00,0x00,0x00};       /* leal 0L(%edi,1),%edi */
  static const char f32_11[] =
    {0x8d,0x74,0x26,0x00,                       /* leal 0(%esi,1),%esi  */
     0x8d,0xbc,0x27,0x00,0x00,0x00,0x00};       /* leal 0L(%edi,1),%edi */
  static const char f32_12[] =
    {0x8d,0xb6,0x00,0x00,0x00,0x00,             /* leal 0L(%esi),%esi   */
     0x8d,0xbf,0x00,0x00,0x00,0x00};            /* leal 0L(%edi),%edi   */
  static const char f32_13[] =
    {0x8d,0xb6,0x00,0x00,0x00,0x00,             /* leal 0L(%esi),%esi   */
     0x8d,0xbc,0x27,0x00,0x00,0x00,0x00};       /* leal 0L(%edi,1),%edi */
  static const char f32_14[] =
    {0x8d,0xb4,0x26,0x00,0x00,0x00,0x00,        /* leal 0L(%esi,1),%esi */
     0x8d,0xbc,0x27,0x00,0x00,0x00,0x00};       /* leal 0L(%edi,1),%edi */
  static const char f32_15[] =
    {0xeb,0x0d,0x90,0x90,0x90,0x90,0x90,        /* jmp .+15; lotsa nops */
     0x90,0x90,0x90,0x90,0x90,0x90,0x90,0x90};

- -- 
- --------------.                        ,-.            444 Castro Street
Ulrich Drepper \    ,-----------------'   \ Mountain View, CA 94041 USA
Red Hat         `--' drepper at redhat.com `---------------------------
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